1. Field of the Invention
The present invention relates to a recording/reproducing optical disk apparatus (recording type optical disk apparatus) for recording or reproducing data by a very small optical spot and a two-layer optical disk used with the recording type optical disk apparatus.
2. Description of the Related Art
In the field of optical disks which record or reproduce data by a very small optical spot, in addition to reproduction only optical disk ROM (Read Only Memory) media on which embossed data pit arrays are formed in advance, CD-Rs (Compact Disc-Recordable) and DVD-Rs (Digital Versatile Disc-Recordable), which are optical disks capable of recording data, have been widely used. Also, CD-RWs (CD-ReWritable) and DVD-RWs (DVD-ReWritable), which can rewrite recorded data, have been used. Moreover, as a next-generation DVD, an optical disk format referred to as an HD DVD-R which can record using a blue light source have been developed. In this description, the optical disks which can record or rewrite data are generically referred to as recording type optical disks.
On the recording type optical disks, a spiral groove track (or a pre-groove) for tracking is formed on an optical disk substrate, and a recording layer composed of multilayer films including an organic material are formed on the optical disk substrate with the spiral groove track. Date is recorded by forming recording pits by condensing a high-power laser beam on the recording layer and partially transforming the recording layer. After the recording, because a servo signal which has similar characteristics to a ROM medium on which embossed data pit arrays are formed can be obtained with a same data format as the ROM medium, it has the advantage that reproduction can be readily performed by a reproduction only driving device.
Further, recently, in such recording type optical disks, optical disk media which realize increase of recording capacity by forming two recording layers and recording or reproducing from a same substrate incident surface have been developed and manufactured. For example, in the DVD-Rs, such two-layer media have been manufactured.
In the reproduction only ROM media, also in the DVD-Rs, the two-layer optical disks have already been widely manufactured. However, in order to allow the recording optical disks to have the two layers, it is necessary to solve their peculiar problems.
Particularly, as shown in FIG. 7, the two-layer optical disk has two recording layers referred to as a zeroth recording layer and a first recording layer. Access to the two recording layers by a laser beam is performed from an incident surface 4 respectively. When a recording is performed to the first recording layer which is relatively far from the incident surface 4, the condensed beam always penetrates the zeroth recording layer. Then, in an unrecorded part 5 and a recorded part 6 of the zeroth recording layer, generally, transmittances are different form each other. Accordingly, if a recording operation is performed to the first recording layer across the recorded pert and the unrecorded part of the zeroth layer, the power of the condensed beam to be reached to the first recording layer is changed during the recording, comes off the optimal condition, and it is difficult to record in a good condition.
To solve this problem, a method that after all of the first recording layer is recorded, a recording on the zeroth recording layer is performed so that the recording on the first recording layer is performed always under the same condition has been described in Japanese Unexamined Patent Application Publication No. 10-26957.
Further, for a two-layer medium of the DVD-R, a format when a data recording is performed onto the first recording layer, the recording is performed through a part of the zeroth recording layer where is in a recorded state has been established.
In the method that the recording on the zeroth recording layer is performed after all of the first recording layer is recorded, a control data recording area for recording control which is generally provided in an inner circumferential part of the disk has to be provided to each of the two recording layers respectively, and the processing is complicated. Moreover, recording state of the overall two-layer optical disk cannot be checked by just reproducing the control data recording area of one of the recording layers.
When data is recorded onto the first recording layer, in the format that the recording is always performed through the recorded part of the zeroth recording layer, in a test zone for optimizing and checking a recording condition set to an inner circumference part or an outer circumference part of the optical disk, because the corresponding zeroth recording layer is not always recorded, an optimum recording condition in the recorded part has to be estimated using the optimum recording condition checked by test recording in the unrecorded state.
Moreover, if an interval between the two recording layers becomes narrow, other problem described below occurs.
That is, as shown in FIG. 8, a recording or reproducing is performed on one recording layer by condensing a laser beam (shown by a solid line), reflection from another recording layer returns to the optical head which is used for recording or reproducing as coherent light (shown by a dotted line). In FIG. 8, in the right-hand side, a case in which the laser beam is condensed on the zeroth recording layer is shown and in substantially center and left-hand side, a case in which the laser beam is condensed on the first recording layer is shown.
When the laser beam is condensed on the zeroth recording layer, the coherent light from the first recording layer becomes to the same reflected light as in the case in which a virtual condensing point appears at back side twice far of the interval between the recording layers, and when the laser beam is condensed on the first recording layer, the coherent light from the zeroth recording layer becomes to the same reflected light as in the case in which a virtual condensing point appears at front side twice far of the interval between the recording layers.
If the interval between the recording layers is wide, because these coherent lights can be considered as reflected lights from the virtual condensing points largely displaced from the actual condensing points, a percentage of the coherent lights which reaches a photodetector in the optical head is small and substantially, there is no problem. However, if the interval between the recording layers becomes narrow and an amount of the coherent light reaches the photodetector becomes to a considerable level, problems occur. A salient problem is the two positions of the condensed beams shown in the right-hand side of the drawing in a case in which the reflected lights are generated at the border parts between the recorded part 6 and the unrecorded part 5. In this case, imbalance of optical power is generated in the coherent light. In the optical head for the recording type optical disk, a push-pull method in which imbalance in the cross section of the reflected light generated from a pre-groove is detected to be a track error signal is employed. Accordingly, if an imbalanced coherent light is received by the optical head, the possibility to be an offset of the track error signal is high. If such offset amount becomes significant, not only the recording operation but also the reproduction operation cannot be stably realized.
Such imbalance of the coherent light can be generated not only at the border between the recorded part and the unrecorded part but as the position of the condensed beam in the left-hand side in the drawing, at the border between a reproduction only pit area 3 which is often provided on an optical disk inner circumference part for control information recording and the unrecorded area 5.
As described above, the known recording type optical disk has the problem that the recording processing on the control data recording area is complicated and in order to know a recording state, it is necessary to access to the control data recording areas of the two recording layers. In addition, other known recording type optical disks have the problem that the optimum recording condition has to be estimated. Moreover, the known recording optical disks has the problem that the narrower the interval between the recording layers becomes, the more affected by the coherent light.